1. Field of the Invention
The present invention relates to amplifiers, and more particularly, to circuits for improving the gain bandwidth product of amplifiers.
2. Description of the Prior Art
The bandwidth of an amplifier can be defined as the difference between the maximum and minimum input frequencies at which a reasonably non-distorted output signal can be obtained. The gain of the amplifier is a measure of the amplification of the output signal relative to the input signal to the amplifier. The product of these two parameters defines a third parameter often referred to as the gain bandwith product. Because the gain bandwidth product (GBWP) is often a constant for a particular amplifier, increasing the gain of the amplifier typically causes a corresponding reduction in the bandwidth of the amplifier. Accordingly, it would be desirable to increase the GBWP of an amplifier in order to increase the gain or bandwidth without having an adverse effect on the other factor.
The GBWP of a typical complementary metal oxide semiconductor field-effect transistor (CMOS) two-stage operational amplifier (FIG. 1) is usually limited by internal capacitances of various transistors within the operational amplifier circuit. These capacitive loads create "poles" in the transfer function of the amplifier, each of which causes an attenuation and phase shift in the output signal at a particular "pole frequency". Due to various design constraints, the pole frequencies of one or more of the transfer function poles is often sufficiently low to adversely affect the GBWP of the amplifier.